Light-emitting device

ABSTRACT

A light-emitting device in which variation in luminance of pixels is suppressed. A light-emitting device includes at least a transistor, a first wiring, a second wiring, a first switch, a second switch, a third switch, a fourth switch, a capacitor, and a light-emitting element. The first wiring and a first electrode of the capacitor are electrically connected to each other through the first switch. A second electrode of the capacitor is connected to a first terminal of the transistor. The second wiring and a gate of the transistor are electrically connected to each other through the second switch. The first electrode of the capacitor and the gate of the transistor are electrically connected to each other through the third switch. The first terminal of the transistor and an anode of the light-emitting element are electrically connected to each other through the fourth switch.

TECHNICAL FIELD

The present invention relates to a light-emitting device in which atransistor is provided in each pixel.

BACKGROUND ART

Since display devices using light-emitting elements have highvisibility, are suitable for reduction in thickness, and do not havelimitations on viewing angles, they have attracted attention as displaydevices that supersede cathode ray tubes (CRTs) and liquid crystaldisplay devices. Specifically proposed structures of active matrixdisplay devices using light-emitting elements are different depending onmanufacturers. In general, a pixel includes at least a light-emittingelement, a transistor that controls input of video signals to the pixel(a switching transistor), and a transistor that controls the amount ofcurrent supplied to the light-emitting element (a driving transistor).

When all the transistors in pixels have the same polarity, it ispossible to omit some of steps for fabricating the transistors, forexample, a step of adding an impurity element imparting one conductivitytype to a semiconductor film. Patent Document 1 discloses alight-emitting element type display in which transistors included inpixels are all n-channel transistors.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2003-195810

DISCLOSURE OF INVENTION

In a light-emitting device, drain current of a driving transistor issupplied to a light-emitting element; thus, when the threshold voltagesof driving transistors vary among pixels, the luminances oflight-emitting elements vary accordingly. Therefore, in order to improvethe image quality of a light-emitting device, it is important to proposea pixel configuration in which a current value of a driving transistorcan be compensated in anticipation of variation in threshold voltage.

In general, a surface of a conductive film used as an anode of alight-emitting element is less likely to be oxidized in the air thanthat of a conductive film used as a cathode of a light-emitting element.In addition, since a conductive film used as an anode of alight-emitting element is generally formed by sputtering, when the anodeis formed over an EL layer containing a light-emitting material, the ELlayer tends to be damaged by sputtering. In view of the above, alight-emitting element in which an anode, an EL layer, and a cathode arestacked in this order can be fabricated through a simple process and caneasily achieve high emission efficiency. However, when an n-channeldriving transistor is used in combination with the above light-emittingelement, a source of the driving transistor is connected to the anode ofthe light-emitting element. In that case, when the voltage between theanode and the cathode of the light-emitting element is increased owingto deterioration of the light-emitting material, the potential of thesource of the driving transistor is increased, whereby the voltagebetween a gate and the source (gate voltage) of the driving transistoris decreased. Accordingly, the drain current of the driving transistor,that is, a current supplied to the light-emitting element is decreased,resulting in a decrease in luminance of the light-emitting element.

In view of the foregoing technical background, an object of oneembodiment of the present invention is to provide a light-emittingdevice in which variation in luminance of pixels caused by variation inthreshold voltage of driving transistors is suppressed. Another objectof one embodiment of the present invention is to provide alight-emitting device in which a decrease in luminance of alight-emitting element caused by deterioration of an EL layer issuppressed.

A light-emitting device according to one embodiment of the presentinvention includes at least a transistor, a first wiring, a secondwiring, a first switch, a second switch, a third switch, a fourthswitch, a capacitor, and a light-emitting element. The first switch hasa function of determining whether electrical continuity is establishedbetween the first wiring and one of a pair of electrodes of thecapacitor. The other of the pair of electrodes of the capacitor isconnected to one of a source and a drain of the transistor. The secondswitch has a function of determining whether electrical continuity isestablished between the second wiring and a gate of the transistor. Thethird switch has a function of determining whether electrical continuityis established between one of the pair of electrodes of the capacitorand the gate of the transistor. The fourth switch has a function ofdetermining whether electrical continuity is established between one ofthe source and the drain of the transistor and an anode of thelight-emitting element.

A light-emitting device according to another embodiment of the presentinvention includes at least a transistor, a first wiring, a secondwiring, a third wiring, a first switch, a second switch, a third switch,a fourth switch, a capacitor, and a light-emitting element. The firstswitch has a function of determining whether electrical continuity isestablished between the first wiring and one of a pair of electrodes ofthe capacitor. The other of the pair of electrodes of the capacitor isconnected to one of a source and a drain of the transistor and an anodeof the light-emitting element. The second switch has a function ofdetermining whether electrical continuity is established between thesecond wiring and a gate of the transistor. The third switch has afunction of determining whether electrical continuity is establishedbetween the one of the pair of electrodes of the capacitor and the gateof the transistor. The fourth switch has a function of determiningwhether electrical continuity is established between the one of thesource and the drain of the transistor and the third wiring.

Note that the switch is an element having a function of controllingsupply of current or potential, and can be an electrical switch or amechanical switch, for example. Specifically, the switch may be atransistor, a diode, or a logic circuit composed of transistors.

In the light-emitting device according to one embodiment of the presentinvention, with the above-described configuration, a potential that ishigher than the threshold voltage of the driving transistor and lowerthan a voltage that is the sum of the threshold voltage and the voltagebetween the source and the drain of the driving transistor can beapplied between the gate and the source of the driving transistor. Whenthe source of the driving transistor is made floating while the abovevoltage is applied, the threshold voltage can be obtained between thegate and the source of the driving transistor. Then, when the voltage ofan image signal is applied to the gate while the source is keptfloating, a voltage that is the sum of the voltage of the image signaland the threshold voltage is applied between the gate and the source ofthe driving transistor. The light-emitting element is supplied with acurrent corresponding to the gate voltage of the driving transistor andexpresses gradation accordingly.

In the light-emitting device according to one embodiment of the presentinvention, a potential that is the sum of the voltage of an image signaland the threshold voltage of the transistor can be applied to the gateelectrode of the transistor; consequently, compensation of the thresholdvoltage and compensation of the potential of the anode can increase theimage quality of the light-emitting device.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are circuit diagrams of pixels;

FIG. 2 is a timing chart illustrating the operation of a pixel;

FIGS. 3A to 3C illustrate the operation of a pixel;

FIG. 4 is a timing chart illustrating the operation of a pixel;

FIGS. 5A to 5C illustrate the operation of a pixel;

FIG. 6 is a top view of a pixel;

FIG. 7 is a cross-sectional view of a pixel;

FIG. 8 is a top view of a pixel;

FIG. 9 is a cross-sectional view of a pixel;

FIG. 10 is a cross-sectional view of pixels;

FIGS. 11A to 11C are cross-sectional views of pixels;

FIG. 12 is a perspective view of a panel;

FIGS. 13A to 13E illustrate electronic devices;

FIGS. 14A to 14E illustrate a structure of an oxide semiconductor;

FIGS. 15A to 15C illustrate a structure of an oxide semiconductor;

FIGS. 16A to 16C illustrate a structure of an oxide semiconductor;

FIG. 17 shows a calculation result; and

FIG. 18 shows a calculation result.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be hereinafter described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description, and it iseasily understood by those skilled in the art that the mode and detailscan be variously changed without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description of the embodiments below.

Note that a light-emitting device in this specification includes, in itscategory, a panel in which a light-emitting element is formed in eachpixel and a module in which an IC or the like including a controller ismounted on the panel.

Embodiment 1

FIG. 1A illustrates an example of the configuration of a pixel 100included in a light-emitting device according to one embodiment of thepresent invention.

The pixel 100 includes transistors 11 to 15, a capacitor 16, and alight-emitting element 17. FIG. 1A shows the case where the transistors11 to 15 are n-channel transistors.

The transistor 12 has a function of determining whether electricalcontinuity is established between a wiring SL and one of a pair ofelectrodes of the capacitor 16. The other of the pair of electrodes ofthe capacitor 16 is connected to one of a source and a drain of thetransistor 11. The transistor 13 has a function of determining whetherelectrical continuity is established between a wiring IL and a gate ofthe transistor 11. The transistor 14 has a function of determiningwhether electrical continuity is established between one of the pair ofelectrodes of the capacitor 16 and the gate of the transistor 11. Thetransistor 15 has a function of determining whether electricalcontinuity is established between one of the source and the drain of thetransistor 11 and an anode of the light-emitting element 17.

In FIG. 1A, the other of the source and the drain of the transistor 11is connected to a wiring VL.

The on/off state of the transistor 12 is determined by the potential ofa wiring G1 connected to a gate of the transistor 12. The on/off stateof the transistor 13 is determined by the potential of the wiring G1connected to a gate of the transistor 13. The on/off state of thetransistor 14 is determined by the potential of a wiring G2 connected toa gate of the transistor 14. The on/off state of the transistor 15 isdetermined by the potential of a wiring G3 connected to a gate of thetransistor 15.

Note that in this specification, the term “connection” means electricalconnection and corresponds to a state in which current, voltage, or apotential can be supplied or transmitted. Therefore, a state ofconnection means not only a state of direct connection but also a stateof indirect connection through an element such as a wiring, a conductivefilm, a resistor, a diode, or a transistor, in which current, voltage,or a potential can be supplied or transmitted.

Even when different components are connected to each other in a circuitdiagram, there is actually a case where one conductive film hasfunctions of a plurality of components, such as a case where part of awiring serves as an electrode. The term “connection” in thisspecification also includes in its category such a case where oneconductive film has functions of a plurality of components.

The light-emitting element 17 includes the anode, a cathode, and an ELlayer provided between the anode and the cathode. The EL layer is formedusing a single layer or plural layers, at least one of which is alight-emitting layer containing a light-emitting substance. From the ELlayer, electroluminescence is obtained by current supplied when apotential difference between the cathode and the anode, using thepotential of the cathode as a reference potential, is higher than orequal to a threshold voltage Vthe of the light-emitting element 17. Aselectroluminescence, there are luminescence (fluorescence) at the timeof returning from a singlet-excited state to a ground state andluminescence (phosphorescence) at the time of returning from atriplet-excited state to a ground state.

Note that the terms “source” and “drain” of a transistor interchangewith each other depending on the polarity of the transistor or thelevels of potentials applied to the source and the drain. In general, inan n-channel transistor, one to which a lower potential is applied iscalled a source, and one to which a higher potential is applied iscalled a drain. In a p-channel transistor, one to which a lowerpotential is supplied is called a drain, and one to which a higherpotential is supplied is called a source. In this specification,although the connection relation of the transistor is sometimesdescribed assuming that the source and the drain are fixed forconvenience, actually, the names of the source and the drain mayinterchange with each other depending on the relation of the potentials.

FIG. 1B illustrates another example of the pixel 100 included in thelight-emitting device according to one embodiment of the presentinvention.

The pixel 100 includes the transistors 11 to 15, the capacitor 16, andthe light-emitting element 17. FIG. 1B shows the case where thetransistors 11 to 15 are n-channel transistors.

The transistor 12 has a function of determining whether electricalcontinuity is established between the wiring SL and one of the pair ofelectrodes of the capacitor 16. The other of the pair of electrodes ofthe capacitor 16 is connected to one of the source and the drain of thetransistor 11 and the anode of the light-emitting element 17. Thetransistor 13 has a function of determining whether electricalcontinuity is established between the wiring IL and the gate of thetransistor 11. The transistor 14 has a function of determining whetherelectrical continuity is established between one of the pair ofelectrodes of the capacitor 16 and the gate of the transistor 11. Thetransistor 15 has a function of determining whether electricalcontinuity is established between one of the source and the drain of thetransistor 11 and a wiring RL and between the anode of thelight-emitting element 17 and the wiring RL. The other of the source andthe drain of the transistor 11 is connected to the wiring VL.

The on/off state of the transistor 12 is determined by the potential ofthe wiring G1 connected to the gate of the transistor 12. The on/offstate of the transistor 13 is determined by the potential of the wiringG1 connected to the gate of the transistor 13. The on/off state of thetransistor 14 is determined by the potential of the wiring G2 connectedto the gate of the transistor 14. The on/off state of the transistor 15is determined by the potential of the wiring G3 connected to the gate ofthe transistor 15.

In FIGS. 1A and 1B, the transistors 11 to 15 each have the gate placedon at least one side of a semiconductor film; alternatively, thetransistors 11 to 15 may have a pair of gates between which thesemiconductor film is sandwiched. When one of the pair of gates and theother one of the pair of gates are regarded as a front gate and a backgate, respectively, the back gate may be floating or may be externallysupplied with a potential. In the latter case, potentials at the samelevel may be applied to the front gate and the back gate, or a fixedpotential such as a ground potential may be applied only to the backgate. By controlling the level of the potential applied to the backgate, the threshold voltage of the transistor can be controlled. Byproviding the back gate, a channel formation region is enlarged and thedrain current can be increased. Moreover, providing the back gatefacilitates formation of a depletion layer in the semiconductor film,which results in lower subthreshold swing.

FIGS. 1A and 1B each show the case where the transistors 11 to 15 aren-channel transistors. When the transistors 11 to 15 have the samepolarity, it is possible to omit some of steps for fabricating thetransistors, for example, a step of adding an impurity element impartingone conductivity type to the semiconductor film. Note that in thelight-emitting device according to one embodiment of the presentinvention, not all the transistors 11 to 15 are necessarily n-channeltransistors. At least the transistor 11 is preferably an n-channeltransistor when the anode of the light-emitting element 17 is connectedto one of a source and a drain of the transistor 15, whereas at leastthe transistor 11 is preferably a p-channel transistor when the cathodeof the light-emitting element 17 is connected to one of the source andthe drain of the transistor 15.

In the case where the transistor 11 operates in a saturation region topass a current therethrough, its channel length or channel width ispreferably larger than those of the transistors 12 to 15. The increasein the channel length or channel width may make the drain current in thesaturation region constant, thereby reducing the kink effect.Alternatively, the increase in the channel length or channel widthallows a large amount of current to flow through the transistor 11 evenin the saturation region.

FIGS. 1A and 1B each illustrate that the transistors 11 to 15 have asingle-gate structure including one gate and one channel formationregion; however, the transistor in the present invention is not limitedto a single-gate transistor. Any or all of the transistors 11 to 15 mayhave a multi-gate structure including a plurality of gates electricallyconnected to each other and a plurality of channel formation regions.

Next, the operation of the pixel 100 illustrated in FIG. 1A will bedescribed.

FIG. 2 is an example of a timing chart showing the potentials of thewirings G1 to G3 and a potential Vdata supplied to the wiring SL; thewirings G1 to G3 and the wiring SL are connected to the pixel 100 inFIG. 1A. Note that the timing chart in FIG. 2 illustrates the case wherethe transistors 11 to 15 are n-channel transistors. As illustrated inFIG. 2, the operation of the pixel 100 in FIG. 1A can be mainly dividedinto a first operation in a first period, a second operation in a secondperiod, and a third operation in a third period.

First, the first operation in the first period is described. In thefirst period, a low-level potential is applied to the wiring G1, alow-level potential is applied to the wiring G2, and a high-levelpotential is applied to the wiring G3. As a result, the transistor 15 isturned on, and the transistors 12 to 14 are turned off

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the cathode of the light-emitting element 17. The potentialVano is higher than a potential that is the sum of the threshold voltageVthe of the light-emitting element 17 and the potential Vcat. Thethreshold voltage Vthe of the light-emitting element 17 is hereinafterassumed to be 0.

FIG. 3A illustrates the operation of the pixel 100 in the first period.In FIG. 3A, the transistors 12 to 15 are represented as switches. In thefirst period, by the above operation, the potential of one of the sourceand the drain of the transistor 11 (illustrated as a node A) becomes thepotential which is the sum of the potential Vcat and the thresholdvoltage Vthe of the light-emitting element 17. In FIG. 3A, the potentialof the node A becomes the potential Vcat because the threshold voltageVthe is assumed to be 0.

Next, the second operation in the second period is described. In thesecond period, a high-level potential is applied to the wiring G1, alow-level potential is applied to the wiring G2, and a low-levelpotential is applied to the wiring G3. As a result, the transistors 12and 13 are turned on, the transistor 14 remains off, and the transistor15 is turned off

During the transition from the first period to the second period, it ispreferable that the potential applied to the wiring G3 be switched froma high-level potential to a low-level potential after the potentialapplied to the wiring G1 is switched from a low-level potential to ahigh-level potential, in which case the potential of the node A can beprevented from being changed by switching of the potential applied tothe wiring G1.

The potential Vano is applied to the wiring VL, and the potential Vcatis applied to the cathode of the light-emitting element 17. A potentialV0 is applied to the wiring IL, and the potential Vdata of an imagesignal is applied to the wiring SL.

Note that the potential V0 is preferably higher than a potential that isthe sum of the potential Vcat, a threshold voltage Vth of the transistor11, and the threshold voltage Vthe of the light-emitting element 17 andis preferably lower than a potential that is the sum of the potentialVano and the threshold voltage Vth of the transistor 11.

FIG. 3B illustrates the operation of the pixel 100 in the second period.In FIG. 3B, the transistors 12 to 15 are represented as switches. In thesecond period, the transistor 11 is turned on since the potential V0 isapplied to the gate of the transistor 11 (illustrated as a node B) bythe above operation. Thus, charge in the capacitor 16 is dischargedthrough the transistor 11, and the potential of the node A, which is thepotential Vcat, starts to rise. Then, when the potential of the node Afinally reaches the potential V0−Vth, that is, when the gate voltage ofthe transistor 11 is decreased to the threshold voltage Vth, thetransistor 11 is turned off. The potential Vdata is applied to oneelectrode of the capacitor 16 (illustrated as a node C).

Next, the third operation in the third period is described. In the thirdperiod, a low-level potential is applied to the wiring G1, a high-levelpotential is applied to the wiring G2, and a high-level potential isapplied to the wiring G3. As a result, the transistors 14 and 15 areturned on, and the transistors 12 and 13 are turned off

During the transition from the second period to the third period, it ispreferable that the potentials applied to the wirings G2 and G3 beswitched from a low-level potential to a high-level potential after thepotential applied to the wiring G1 is switched from a high-levelpotential to a low-level potential, in which case the potential of thenode A can be prevented from being changed by switching of the potentialapplied to the wiring G1.

The potential Vano is applied to the wiring VL, and the potential Vcatis applied to the cathode of the light-emitting element 17.

FIG. 3C illustrates the operation of the pixel 100 in the third period.In FIG. 3C, the transistors 12 to 15 are represented as switches. In thethird period, the gate voltage of the transistor 11 becomes Vdata−V0+Vthsince the potential Vdata is applied to the node B by the aboveoperation. That is, the gate voltage of the transistor 11 can be thevalue to which the threshold voltage Vth is added. Consequently,variation in the threshold voltage Vth of the transistors 11 can beprevented from adversely affecting the value of a current supplied tothe light-emitting elements 17. Alternatively, even if the transistor 11deteriorates and the threshold voltage Vth is changed, the change in thethreshold voltage Vth can be prevented from adversely affecting thevalue of a current supplied to the light-emitting element 17. Therefore,display unevenness can be reduced, and high-quality images can bedisplayed.

Next, the operation of the pixel 100 illustrated in FIG. 1B will bedescribed.

FIG. 4 is an example of a timing chart showing the potentials of thewirings G1 to G3 and the potential Vdata supplied to the wiring SL; thewirings G1 to G3 and the wiring SL are connected to the pixel 100 inFIG. 1B. Note that the timing chart in FIG. 4 illustrates the case wherethe transistors 11 to 15 are n-channel transistors. As illustrated inFIG. 4, the operation of the pixel 100 in FIG. 1B can be mainly dividedinto a first operation in a first period, a second operation in a secondperiod, and a third operation in a third period.

First, the first operation in the first period is described. In thefirst period, a low-level potential is applied to the wiring G1, alow-level potential is applied to the wiring G2, and a high-levelpotential is applied to the wiring G3. As a result, the transistor 15 isturned on, and the transistors 12 to 14 are turned off

The potential Vano is applied to the wiring VL, and the potential Vcatis applied to the cathode of the light-emitting element 17. As describedabove, the potential Vano is higher than the potential which is the sumof the threshold voltage Vthe of the light-emitting element 17 and thepotential Vcat. A potential V1 is applied to the wiring RL. Thepotential V1 is preferably lower than a potential that is the sum of thepotential Vcat and the threshold voltage Vthe of the light-emittingelement 17. With the potential V1 set in the above range, a current canbe prevented from flowing through the light-emitting element 17 in thefirst period.

FIG. 5A illustrates the operation of the pixel 100 in the first period.In FIG. 5A, the transistors 12 to 15 are represented as switches. In thefirst period, the potential V1 is applied to one of the source and thedrain of the transistor 11 (illustrated as the node A) by the aboveoperation.

Next, the second operation in the second period is described. In thesecond period, a high-level potential is applied to the wiring G1, alow-level potential is applied to the wiring G2, and a low-levelpotential is applied to the wiring G3. As a result, the transistors 12and 13 are turned on, the transistor 14 remains off, and the transistor15 is turned off

During the transition from the first period to the second period, it ispreferable that the potential applied to the wiring G3 be switched froma high-level potential to a low-level potential after the potentialapplied to the wiring G1 is switched from a low-level potential to ahigh-level potential, in which case the potential of the node A can beprevented from being changed by switching of the potential applied tothe wiring G1.

The potential Vano is applied to the wiring VL, and the potential Vcatis applied to the cathode of the light-emitting element 17. Thepotential V0 is applied to the wiring IL, and the potential Vdata of animage signal is applied to the wiring SL. As described above, thepotential V0 is preferably higher than the potential which is the sum ofthe potential Vcat, the threshold voltage Vth of the transistor 11, andthe threshold voltage Vthe of the light-emitting element 17 and ispreferably lower than the potential which is the sum of the potentialVano and the threshold voltage Vth of the transistor 11. Note thatunlike in the pixel 100 in FIG. 1A, the anode of the light-emittingelement 17 is connected to one of the source and the drain of thetransistor 11 in the pixel 100 in FIG. 1B. For that reason, thepotential V0 in the pixel 100 in FIG. 1B is preferably set lower thanthat in the pixel 100 in FIG. 1A in order not to increase the value ofcurrent that is supplied to the light-emitting element 17 in the secondperiod.

FIG. 5B illustrates the operation of the pixel 100 in the second period.In FIG. 5B, the transistors 12 to 15 are represented as switches. In thesecond period, the transistor 11 is turned on since the potential V0 isapplied to the gate of the transistor 11 (illustrated as the node B) bythe above operation. Thus, charge in the capacitor 16 is dischargedthrough the transistor 11, and the potential of the node A, which is thepotential V1, starts to rise. Then, when the potential of the node Afinally reaches the potential V0−Vth, that is, when the gate voltage ofthe transistor 11 is decreased to the threshold voltage Vth, thetransistor 11 is turned off. The potential Vdata is applied to oneelectrode of the capacitor 16 (illustrated as the node C).

Next, the third operation in the third period is described. In the thirdperiod, a low-level potential is applied to the wiring G1, a high-levelpotential is applied to the wiring G2, and a low-level potential isapplied to the wiring G3. As a result, the transistor 14 is turned on,the transistors 12 and 13 are turned off, and the transistor 15 remainsoff

During the transition from the second period to the third period, it ispreferable that the potential applied to the wiring G2 be switched froma low-level potential to a high-level potential after the potentialapplied to the wiring G1 is switched from a high-level potential to alow-level potential, in which case the potential of the node A can beprevented from being changed by switching of the potential applied tothe wiring G1.

The potential Vano is applied to the wiring VL, and the potential Vcatis applied to the cathode of the light-emitting element 17.

FIG. 5C illustrates the operation of the pixel 100 in the third period.In FIG. 5C, the transistors 12 to 15 are represented as switches. In thethird period, the gate voltage of the transistor 11 becomes Vdata−V0+Vthsince the potential Vdata is applied to the node B by the aboveoperation. That is, the gate voltage of the transistor 11 can be thevalue to which the threshold voltage Vth is added. Consequently,variation in the threshold voltage Vth of the transistors 11 can beprevented from adversely affecting the value of a current supplied tothe light-emitting elements 17. Alternatively, even if the transistor 11deteriorates and the threshold voltage Vth is changed, the change in thethreshold voltage Vth can be prevented from adversely affecting thevalue of a current supplied to the light-emitting element 17. Therefore,display unevenness can be reduced, and high-quality images can bedisplayed.

In the light-emitting element type display disclosed in Patent Document1, a gate and a drain of a transistor (Tr12) for supplying current to anorganic EL element are electrically connected to each other to obtainthe threshold voltage. For that reason, when the transistor (Tr12) is anormally-on transistor, the potential of the source of the transistor(Tr12) is never higher than that of the gate. It is therefore difficultto obtain the threshold voltage when the transistor (Tr12) is anormally-on transistor.

In contrast, in the light-emitting device according to one embodiment ofthe present invention, which includes the pixel illustrated in FIG. 1Aor FIG. 1B, the other of the source and the drain of the transistor 11is electrically separated from the gate of the transistor 11, so thattheir potentials can be individually controlled. Accordingly, in thesecond operation, the potential of the other of the source and the drainof the transistor 11 can be set higher than a value that is the sum ofthe potential of the gate of the transistor 11 and the threshold voltageVth. Therefore, when the transistor 11 is a normally-on transistor, thatis, when the threshold voltage Vth is negative, charge can beaccumulated in the capacitor 16 until the potential of the source of thetransistor 11 becomes higher than the potential V0 of the gate of thetransistor 11. Thus, in the light-emitting device according to oneembodiment of the present invention, even when the transistor 11 is anormally-on transistor, the threshold voltage can be obtained in thesecond operation, and in the third operation, the gate voltage of thetransistor 11 can be set at the value to which the threshold voltage Vthis added.

Therefore, in the light-emitting device according to one embodiment ofthe present invention, display unevenness can be reduced andhigh-quality images can be displayed even if the transistor 11 includinga semiconductor film containing an oxide semiconductor, for example,becomes normally on.

Embodiment 2

FIG. 6 illustrates an example of a top view of the pixel illustrated inFIG. 1A. Note that in the top view of the pixel in FIG. 6, insulatingfilms are omitted in order to clearly show the layout of the pixel.Further, in the top view of the pixel in FIG. 6, the anode, the ELlayer, and the cathode are omitted in order to clearly show the layoutof the transistors and the capacitor included in the pixel.

FIG. 7 is a cross-sectional view along dashed lines A1-A2 and A3-A4 inthe top view in FIG. 6.

The transistor 12 includes, over a substrate 800 having an insulatingsurface, a conductive film 801 functioning as a gate, a gate insulatingfilm 802 over the conductive film 801, a semiconductor film 803positioned over the gate insulating film 802 to overlap with theconductive film 801, and conductive films 804 and 805 that arepositioned over the semiconductor film 803 and function as a source anda drain. The conductive film 801 also functions as the wiring G1. Theconductive film 804 also functions as the wiring SL.

The transistor 13 includes, over the substrate 800 having an insulatingsurface, the conductive film 801 functioning as a gate, the gateinsulating film 802 over the conductive film 801, a semiconductor film806 positioned over the gate insulating film 802 to overlap with theconductive film 801, and conductive films 807 and 808 that arepositioned over the semiconductor film 806 and function as a source anda drain. The conductive film 807 is connected to a conductive film 809functioning as the wiring IL through a contact hole.

The transistor 14 includes, over the substrate 800 having an insulatingsurface, a conductive film 810 functioning as a gate, the gateinsulating film 802 over the conductive film 810, a semiconductor film811 positioned over the gate insulating film 802 to overlap with theconductive film 810, and the conductive films 805 and 808 that arepositioned over the semiconductor film 811 and function as a source anda drain. The conductive film 810 also functions as the wiring G2.

The transistor 11 includes, over the substrate 800 having an insulatingsurface, a conductive film 812 functioning as a gate, the gateinsulating film 802 over the conductive film 812, a semiconductor film813 positioned over the gate insulating film 802 to overlap with theconductive film 812, and conductive films 814 and 815 that arepositioned over the semiconductor film 813 and function as a source anda drain. The conductive film 812 is connected to the conductive film808. The conductive film 814 also functions as the wiring VL.

The transistor 15 includes, over the substrate 800 having an insulatingsurface, a conductive film 816 functioning as a gate, the gateinsulating film 802 over the conductive film 816, a semiconductor film817 positioned over the gate insulating film 802 to overlap with theconductive film 816, and the conductive film 815 and a conductive film818 that are positioned over the semiconductor film 817 and function asa source and a drain. The conductive film 816 also functions as thewiring G3.

The capacitor 16 includes, over the substrate 800 having an insulatingsurface, a conductive film 819, the gate insulating film 802 over theconductive film 819, and the conductive film 815 positioned over thegate insulating film 802 to overlap with the conductive film 819. Theconductive film 819 is connected to the conductive film 805.

An insulating film 820 is formed over the conductive films 804, 805,807, 808, 814, 815, and 818. A conductive film 822 functioning as ananode is formed over the insulating film 821. The conductive film 822 isconnected to the conductive film 818 through a contact hole 823 formedin the insulating films 820 and 821.

An insulating film 824 having an opening where part of the conductivefilm 822 is exposed is provided over the insulating film 821. An ELlayer 825 and a conductive film 826 functioning as a cathode are stackedin this order over the part of the conductive film 822 and theinsulating film 824. A region where the conductive film 822, the ELlayer 825, and the conductive film 826 overlap with one anothercorresponds to the light-emitting element 17.

FIG. 8 illustrates another example of a top view of the pixelillustrated in FIG. 1A. Note that in the top view of the pixel in FIG.8, insulating films are omitted in order to clearly show the layout ofthe pixel. Further, in the top view of the pixel in FIG. 8, the anode,the EL layer, and the cathode are omitted in order to clearly show thelayout of the transistors and the capacitor included in the pixel.

FIG. 9 is a cross-sectional view along dashed lines A1-A2 and A3-A4 inthe top view in FIG. 8.

The transistor 12 includes, over a substrate 900 having an insulatingsurface, a semiconductor film 901, a gate insulating film 902 over thesemiconductor film 901, a conductive film 903 that is positioned overthe gate insulating film 902 to overlap with the semiconductor film 901and functions as a gate, and conductive films 904 and 905 connected to asource and a drain included in the semiconductor film 901. Theconductive film 903 also functions as the wiring G1. The conductive film904 also functions as the wiring SL.

The transistor 13 includes, over the substrate 900 having an insulatingsurface, a semiconductor film 906, the gate insulating film 902 over thesemiconductor film 906, the conductive film 903 that is positioned overthe gate insulating film 902 to overlap with the semiconductor film 906and functions as a gate, and conductive films 907 and 908 connected to asource and a drain included in the semiconductor film 906. Theconductive film 907 is connected to a conductive film 909 functioning asthe wiring IL through a contact hole.

The transistor 14 includes, over the substrate 900 having an insulatingsurface, the semiconductor film 901, the gate insulating film 902 overthe semiconductor film 901, a conductive film 911 that is positionedover the gate insulating film 902 to overlap with the semiconductor film901 and functions as a gate, and the conductive films 905 and 908connected to a source and a drain included in the semiconductor film901. The conductive film 911 also functions as the wiring G2. Note thatin FIG. 8, the transistors 12 and 14 share one semiconductor film 901;alternatively, the transistors 12 and 14 may include differentsemiconductor films.

The transistor 11 includes, over the substrate 900 having an insulatingsurface, a semiconductor film 912, the gate insulating film 902 over thesemiconductor film 912, a conductive film 913 that is positioned overthe gate insulating film 902 to overlap with the semiconductor film 912and functions as a gate, and a conductive film 914 connected to a sourceor a drain included in the semiconductor film 912. The conductive film913 is connected to the conductive film 908. The conductive film 914also functions as the wiring VL.

The transistor 15 includes, over the substrate 900 having an insulatingsurface, the semiconductor film 912, the gate insulating film 902 overthe semiconductor film 912, a conductive film 915 that is positionedover the gate insulating film 902 to overlap with the semiconductor film912 and functions as a gate, and a conductive film 916 connected to asource or a drain included in the semiconductor film 912. The conductivefilm 915 also functions as the wiring G3.

The capacitor 16 includes, over the substrate 900 having an insulatingsurface, the semiconductor film 912, the gate insulating film 902 overthe semiconductor film 912, and a conductive film 917 positioned overthe gate insulating film 902 to overlap with the semiconductor film 912.The conductive film 917 is connected to the conductive film 905.

An insulating film 920 is formed over the conductive films 904, 905,907, 908, 914, and 916. A conductive film 921 functioning as an anode isprovided over the insulating film 920. The conductive film 921 isconnected to the conductive film 916 through a contact hole 922 formedin the insulating film 920.

An insulating film 923 having an opening where part of the conductivefilm 921 is exposed is provided over the insulating film 920. An ELlayer 924 and a conductive film 925 functioning as a cathode are stackedin this order over the part of the conductive film 921 and theinsulating film 923. A region where the conductive film 921, the ELlayer 924, and the conductive film 925 overlap with one anothercorresponds to the light-emitting element 17.

In one embodiment of the present invention, the transistors 11 to 15 mayinclude a semiconductor film containing an amorphous, microcrystalline,polycrystalline, or single crystal semiconductor (e.g., silicon orgermanium), or a semiconductor film containing a wide bandgapsemiconductor (e.g., an oxide semiconductor).

When the semiconductor films of the transistors 11 to 15 are formedusing an amorphous, microcrystalline, polycrystalline, or single crystalsemiconductor (e.g., silicon or germanium), impurity regions functioningas a source and a drain are formed by addition of an impurity elementimparting one conductivity type to the semiconductor films. For example,an impurity region having n-type conductivity can be formed by additionof phosphorus or arsenic to the semiconductor film. Further, an impurityregion having p-type conductivity can be formed by addition of boron,for instance, to the semiconductor film.

In the case where an oxide semiconductor is used for the semiconductorfilms of the transistors 11 to 15, a dopant may be added to thesemiconductor films to form impurity regions functioning as a source anda drain. The dopant can be added by ion implantation. Examples of thedopant are a rare gas such as helium, argon, and xenon; and a Group 15element such as nitrogen, phosphorus, arsenic, and antimony. Forexample, when nitrogen is used as the dopant, the concentration ofnitrogen atoms in the impurity region is preferably from 5×10¹⁹/cm³ to1×10²² /cm³.

As a silicon semiconductor, any of the following can be used, forexample: amorphous silicon formed by sputtering or vapor phase growthsuch as plasma CVD, polycrystalline silicon obtained in such a mannerthat amorphous silicon is crystallized by laser annealing or the like,and single crystal silicon obtained in such a manner that a surfaceportion of a single crystal silicon wafer is separated by implantationof hydrogen ions or the like into the silicon wafer.

Examples of the oxide semiconductor are indium oxide; tin oxide; zincoxide; two-component metal oxides such as In—Zn-based oxide, Sn—Zn-basedoxide, Al—Zn-based oxide, Zn—Mg-based oxide, Sn—Mg-based oxide,In—Mg-based oxide, and In—Ga-based oxide; three-component metal oxidessuch as In—Ga—Zn-based oxide (also referred to as IGZO), In—Al—Zn-basedoxide, In—Sn—Zn-based oxide, Sn—Ga—Zn-based oxide, Al—Ga—Zn-based oxide,Sn—Al—Zn-based oxide, In—Hf—Zn-based oxide, In—La—Zn-based oxide,In—Ce—Zn-based oxide, In—Pr—Zn-based oxide, In—Nd—Zn-based oxide,In—Sm—Zn-based oxide, In—Eu—Zn-based oxide, In—Gd—Zn-based oxide,In—Tb—Zn-based oxide, In—Dy—Zn-based oxide, In—Ho—Zn-based oxide,In—Er—Zn-based oxide, In—Tm—Zn-based oxide, In—Yb—Zn-based oxide, andIn—Lu—Zn-based oxide; and four-component metal oxides such asIn—Sn—Ga—Zn-based oxide, In—Hf—Ga—Zn-based oxide, In—Al—Ga—Zn-basedoxide, In—Sn—Al—Zn-based oxide, In—Sn—Hf—Zn-based oxide, andIn—Hf—Al—Zn-based oxide.

For example, an In—Ga—Zn-based oxide refers to an oxide containing In,Ga, and Zn, and there is no limitation on the ratio of In, Ga, and Zn.The In—Ga—Zn-based oxide may contain a metal element other than In, Ga,and Zn.

A material represented by InMO₃(ZnO)_(m) (m>0 and m is not an integer)may be used as the oxide semiconductor. Note that M represents one ormore metal elements selected from Ga, Fe, Mn, and Co. Alternatively, amaterial expressed by In₂SnO₅(ZnO)_(n)(n>0 and n is a natural number)may be used as the oxide semiconductor.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn =2:2:1 (=2/5:2/5:1/5), or anoxide with an atomic ratio close to the above atomic ratios can be used.Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn =2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8) or an oxide with an atomic ratio close to theabove atomic ratios may be used.

As a stabilizer for reducing variation in electric characteristics oftransistors including the oxide semiconductor, tin (Sn), hafnium (Hf),aluminum (Al), zirconium (Zr), and/or titanium (Ti) is/are preferablycontained. As another stabilizer, one or plural kinds of lanthanoid suchas lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium(Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), andlutetium (Lu) may be contained.

Note that a purified oxide semiconductor (purified OS) obtained byreduction of impurities such as moisture or hydrogen which serves as anelectron donor (donor) and by reduction of oxygen defects is an i-type(intrinsic) semiconductor or a substantially i-type semiconductor.Therefore, a transistor including the purified oxide semiconductor hasextremely low off-state current. The band gap of the oxide semiconductoris 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV ormore. With the use of an oxide semiconductor film that is highlypurified by a sufficient decrease in the concentration of impuritiessuch as moisture and hydrogen and a reduction of oxygen defects, theoff-state current of a transistor can be decreased.

Specifically, various experiments can prove low off-state current of atransistor in which a purified oxide semiconductor is used for asemiconductor film. For example, the off-state current of even atransistor with a channel width of 1×10⁶ μm and a channel length of 10μm can be less than or equal to the measurement limit of a semiconductorparameter analyzer, that is, less than or equal to 1×10⁻¹³ A when thevoltage between a source electrode and a drain electrode (drain voltage)ranges from 1 V to 10 V. In that case, the off-state currentcorresponding to a value obtained by dividing the off-state current bythe channel width of the transistor is 100 zA/μm or less. In addition,the off-state current was measured using a circuit in which a capacitorand a transistor were connected to each other and charge flowing into orfrom the capacitor was controlled by the transistor. For themeasurement, the transistor in which a channel formation region isformed in a purified oxide semiconductor film was used, and theoff-state current of the transistor was measured from a change in theamount of charge of the capacitor per unit time. As a result, it isfound that lower off-state current of several tens of yoctoamperes permicrometer (yA/μm) can be obtained when the voltage between the sourceelectrode and the drain electrode of the transistor is 3 V.Consequently, the off-state current of the transistor in which thechannel formation region is formed in the purified oxide semiconductorfilm is significantly lower than that of a transistor using crystallinesilicon.

Unless otherwise specified, in the case of an n-channel transistor, theoff-state current in this specification is a current that flows betweena source and a drain when the potential of a gate is lower than or equalto 0 with the potential of the source as a reference potential while thepotential of the drain is higher than those of the source and the gate.Moreover, in the case of a p-channel transistor, the off-state currentin this specification is a current that flows between a source and adrain when the potential of a gate is higher than or equal to 0 with thepotential of the source as a reference potential while the potential ofthe drain is lower than those of the source and the gate.

For example, the oxide semiconductor film can be formed by sputteringusing a target including indium (In), gallium (Ga), and zinc (Zn). Whenan In—Ga—Zn-based oxide semiconductor film is formed by sputtering, itis preferable to use an In—Ga—Zn-based oxide target having an atomicratio of In:Ga:Zn =1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or 3:1:4. When anoxide semiconductor film is formed using an In—Ga—Zn-based oxide targethaving the aforementioned atomic ratio, a polycrystal or ac-axis-aligned crystal (CAAC) is likely to be formed. The filling rateof the target including In, Ga, and Zn is 90% or higher and 100% orlower, preferably 95% or higher and lower than 100%. With the use of thetarget with high filling rate, a dense oxide semiconductor film isformed.

In the case where an In—Zn-based oxide material is used as the oxidesemiconductor, the atomic ratio of metal elements in a target to be usedis In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO =25:1 to 1:4 in amolar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio(In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=15:1to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio).For example, in a target used for forming an oxide semiconductor filmcontaining an In—Zn-based oxide with an atomic ratio of In:Zn:O=X:Y:Z,the relation of Z>1.5X+Y is satisfied. The mobility can be increased bykeeping the ratio of Zn within the above range.

Specifically, the oxide semiconductor film may be formed as follows: thesubstrate is held in a treatment chamber with pressure reduced, asputtering gas from which hydrogen and moisture are removed isintroduced while residual moisture in the treatment chamber is removed,and the above-described target is used. The substrate temperature duringfilm formation may be from 100° C. to 600° C., preferably from 200° C.to 400° C. By forming the oxide semiconductor film while the substrateis heated, the concentration of impurities included in the formed oxidesemiconductor film can be reduced. In addition, damage by sputtering canbe reduced. In order to remove remaining moisture in the treatmentchamber, an entrapment vacuum pump is preferably used. For example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. The evacuation unit may be a turbo pump provided with a cold trap.In the deposition chamber which is evacuated with the cryopump, forexample, a hydrogen atom and a compound containing a hydrogen atom, suchas water (H₂O) (preferably, a compound containing a carbon atom as well)are removed, whereby the impurity concentration in the oxidesemiconductor film formed in the chamber can be reduced.

Note that the oxide semiconductor film formed by sputtering or the likesometimes contains a large amount of moisture or hydrogen (including ahydroxyl group) as impurities. Moisture and hydrogen easily form a donorlevel and thus serve as impurities in the oxide semiconductor. In onemode of the present invention, in order to reduce impurities such asmoisture or hydrogen in the oxide semiconductor film (in order toperform dehydration or dehydrogenation), the oxide semiconductor film issubjected to heat treatment in a reduced-pressure atmosphere, an inertgas atmosphere of nitrogen, a rare gas, or the like, an oxygen gasatmosphere, or ultra-dry air (the moisture amount is 20 ppm (−55° C. byconversion into a dew point) or less, preferably 1 ppm or less, furtherpreferably 10 ppb or less in the case where measurement is performed bya dew point meter in a cavity ring-down laser spectroscopy (CRDS)method).

By performing heat treatment on the oxide semiconductor film, moistureor hydrogen in the oxide semiconductor film can be eliminated.Specifically, heat treatment may be performed at a temperature higherthan or equal to 250° C. and lower than or equal to 750° C., preferablyhigher than or equal to 400° C. and lower than the strain point of thesubstrate. For example, heat treatment may be performed at 500° C. forapproximately 3 to 6 minutes. When an RTA method is used for the heattreatment, dehydration or dehydrogenation can be performed in a shorttime; therefore, treatment can be performed even at a temperature higherthan the strain point of a glass substrate.

Note that in some cases, the heat treatment makes oxygen released fromthe oxide semiconductor film and an oxygen defect is formed in the oxidesemiconductor film. To prevent an oxygen defect, an insulating filmincluding oxygen is used as an insulating film in contact with the oxidesemiconductor film, such as a gate insulating film, in one embodiment ofthe present invention. Then, heat treatment is performed after formationof the insulating film including oxygen, so that oxygen is supplied fromthe insulating film to the oxide semiconductor film. With the abovestructure, oxygen defects serving as donors can be reduced in the oxidesemiconductor film and the stoichiometric composition of the oxidesemiconductor included in the oxide semiconductor film can be satisfied.It is preferable that the proportion of oxygen in the oxidesemiconductor film is higher than that in the stoichiometriccomposition. As a result, the oxide semiconductor film can be madesubstantially i-type and variation in electrical characteristics of thetransistors due to oxygen defects can be reduced; thus, electricalcharacteristics can be improved.

The heat treatment for supplying oxygen to the semiconductor film isperformed in a nitrogen atmosphere, ultra-dry air, or a rare gas (e.g.,argon or helium) atmosphere preferably at temperatures ranging from 200°C. to 400° C., for example, from 250° C. to 350° C. It is preferablethat the water content in the gas be 20 ppm or less, preferably 1 ppm orless, further preferably 10 ppb or less.

Further, the oxide semiconductor may be either amorphous or crystalline.In the latter case, the oxide semiconductor may be single crystal orpolycrystalline, or may have a structure in which part of the oxidesemiconductor is crystalline, an amorphous structure including acrystalline portion, or a non-amorphous structure. As an example of sucha partly crystalline structure, an oxide semiconductor including acrystal with c-axis alignment (also referred to as a c-axis-alignedcrystalline oxide semiconductor (CAAC-OS)), which has a triangular orhexagonal atomic arrangement when seen from the direction of an a-bplane, a surface, or an interface, may be used. In the crystal, metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner along the c-axis when seen from thedirection perpendicular to the c-axis, and the direction of the a-axisor the b-axis is varied in the a-b plane (the crystal rotates around thec-axis).

In a broad sense, a CAAC-OS means a non-single-crystal oxide including aphase which has a triangular, hexagonal, regular triangular, or regularhexagonal atomic arrangement when seen from the direction perpendicularto the a-b plane and in which metal atoms are arranged in a layeredmanner or metal atoms and oxygen atoms are arranged in a layered mannerwhen seen from the direction perpendicular to the c-axis direction.

The CAAC-OS is not single crystal, but this does not mean that CAAC-OSis composed of only an amorphous portion. Although the CAAC-OS includesa crystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

Nitrogen may be substituted for part of oxygen which is a constituent ofthe CAAC-OS. The c-axes of the crystalline portions included in theCAAC-OS may be aligned in one direction (e.g., a direction perpendicularto a surface of a substrate over which the CAAC-OS is formed or asurface of the CAAC-OS). Alternatively, the normals of the a-b planes ofindividual crystalline portions included in the CAAC-OS may be alignedin one direction (e.g., the direction perpendicular to a surface of asubstrate over which the CAAC-OS is formed or a surface of the CAAC-OS).

The CAAC-OS is a conductor, a semiconductor, or an insulator, whichdepends on its composition or the like. The CAAC-OS transmits or doesnot transmit visible light depending on its composition or the like.

An example of such a CAAC-OS is an oxide which is formed into a filmshape and has a triangular or hexagonal atomic arrangement when observedfrom the direction perpendicular to a surface of the film or a surfaceof a substrate where the film is formed, and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms (ornitrogen atoms) are arranged in a layered manner when a cross section ofthe film is observed.

An example of a crystal structure of the CAAC-OS will be described indetail with reference to FIGS. 14A to 14E, FIGS. 15A to 15C, and FIGS.16A to 16C. In FIGS. 14A to 14E, FIGS. 15A to 15C, and FIGS. 16A to 16C,the vertical direction corresponds to the c-axis direction and a planeperpendicular to the c-axis direction corresponds to the a-b plane,unless otherwise specified. When the expressions “an upper half” and “alower half” are simply used, they refer to an upper half above the a-bplane and a lower half below the a-b plane (an upper half and a lowerhalf with respect to the a-b plane). In FIGS. 14A to 14E, O surroundedby a circle represents tetracoordinate O and O surrounded by a doublecircle represents tricoordinate O.

FIG. 14A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 14A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of the upper half andthe lower half in FIG. 14A. In the small group illustrated in FIG. 14A,electric charge is 0.

FIG. 14B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One tetracoordinate O atom exists in each of the upper half andthe lower half in FIG. 14B. An In atom can also have the structureillustrated in FIG. 14B because an In atom can have five ligands. In thesmall group illustrated in FIG. 14B, electric charge is 0.

FIG. 14C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 14C,one tetracoordinate O atom exists in the upper half and threetetracoordinate O atoms exist in the lower half Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 14C. In thesmall group illustrated in FIG. 14C, electric charge is 0.

FIG. 14D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. Threetetracoordinate O atoms exist in each of the upper half and the lowerhalf in FIG. 14D. In the small group illustrated in FIG. 14D, electriccharge is +1.

FIG. 14E illustrates a small group including two Zn atoms. Onetetracoordinate O atom exists in each of the upper half and the lowerhalf in FIG. 14E. In the small group illustrated in FIG. 14E, electriccharge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 14A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom in FIG. 14B has oneproximate Ga atom in the downward direction, and the one O atom in thelower half has one proximate Ga atom in the upward direction. The one Oatom in the upper half with respect to the tetracoordinate Zn atom inFIG. 14C has one proximate Zn atom in the downward direction, and thethree O atoms in the lower half each have three proximate Zn atoms inthe upward direction. In this manner, the number of the tetracoordinateO atoms above the metal atom is equal to the number of the metal atomsproximate to and below each of the tetracoordinate O atoms. Similarly,the number of the tetracoordinate O atoms below the metal atom is equalto the number of the metal atoms proximate to and above each of thetetracoordinate O atoms. Since the coordination number of thetetracoordinate O atom is 4, the sum of the number of the metal atomsproximate to and below the O atom and the number of the metal atomsproximate to and above the O atom is 4. Accordingly, when the sum of thenumber of tetracoordinate O atoms above a metal atom and the number oftetracoordinate O atoms below another metal atom is 4, the two kinds ofsmall groups including the metal atoms can be bonded to each other. Forexample, in the case where the hexacoordinate metal (In or Sn) atom isbonded through three tetracoordinate O atoms in the lower half, it isbonded to the pentacoordinate metal (Ga or In) atom or thetetracoordinate metal (Zn) atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through a tetracoordinate O atom in the c-axisdirection. In addition to the above, a medium group can be formed bycombining a plurality of small groups so that the total electric chargeof the layered structure is 0.

FIG. 15A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn-based oxide. FIG. 15B illustrates a large groupincluding three medium groups. FIG. 15C illustrates an atomicarrangement in the case where the layered structure in FIG. 15B isobserved from the c-axis direction.

In FIG. 15A, a tricoordinate O atom is omitted for simplicity, and atetracoordinate O atom is illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of the upper half and the lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, inFIG. 15A, one tetracoordinate O atom existing in each of the upper halfand the lower half with respect to an In atom is denoted by circled 1.FIG. 15A also illustrates a Zn atom proximate to one tetracoordinate Oatom in the lower half and three tetracoordinate O atoms in the upperhalf, and a Zn atom proximate to one tetracoordinate O atom in the upperhalf and three tetracoordinate O atoms in the lower half

In the medium group included in the layered structure of theIn—Sn—Zn-based oxide in FIG. 15A, in the order starting from the top, aSn atom proximate to three tetracoordinate O atoms in each of the upperhalf and the lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of the upper half and the lower half, theIn atom is bonded to a Zn atom proximate to three tetracoordinate Oatoms in the upper half, the Zn atom is bonded to an In atom proximateto three tetracoordinate O atoms in each of the upper half and the lowerhalf through one tetracoordinate O atom in the lower half with respectto the Zn atom, the In atom is bonded to a small group that includes twoZn atoms and is proximate to one tetracoordinate O atom in the upperhalf, and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of the upper half and the lower halfthrough one tetracoordinate O atom in the lower half with respect to thesmall group. A plurality of such medium groups are bonded, so that alarge group is formed.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly,electric charge in a small group including a Sn atom is +1. Therefore,electric charge of −1, which cancels +1, is needed to form a layeredstructure including a Sn atom. An example of a structure having electriccharge of −1 is the small group including two Zn atoms as illustrated inFIG. 14E. For example, with one small group including two Zn atoms,electric charge of one small group including a Sn atom can be cancelled,so that the total electric charge of the layered structure can be 0.

Specifically, be repeating the large group illustrated in FIG. 15B, anIn—Sn—Zn-based oxide crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn-based oxide can be expressedas a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or a naturalnumber).

The above rule also applies to the following oxides: a four-componentmetal oxide such as an In—Sn—Ga—Zn-based oxide; a three-component metaloxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), anIn—Al—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, aSn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide,an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-basedoxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, anIn—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide,an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-basedoxide, an In—Yb—Zn-based oxide, and an In—Lu—Zn-based oxide; atwo-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-basedoxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide,an In—Mg-based oxide, and an In—Ga-based oxide.

As an example, FIG. 16A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn-based oxide.

In the medium group included in the layered structure of theIn—Ga—Zn-based oxide in FIG. 16A, in the order starting from the top, anIn atom proximate to three tetracoordinate O atoms in each of the upperhalf and the lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in the upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of the upper halfand the lower half through three tetracoordinate O atoms in the lowerhalf with respect to the Zn atom, and the Ga atom is bonded to an Inatom proximate to three tetracoordinate O atoms in each of the upperhalf and the lower half through one tetracoordinate O atom in the lowerhalf with respect to the Ga atom. A plurality of such medium groups arebonded, so that a large group is formed.

FIG. 16B illustrates a large group consisting of three medium groups.FIG. 16C illustrates an atomic arrangement in the case where the layeredstructure in FIG. 16B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) Inatom, electric charge of a (tetracoordinate) Zn atom, and electriccharge of a (pentacoordinate) Ga atom are +3, +2, +3, respectively,electric charge of a small group including any of an In atom, a Zn atom,and a Ga atom is 0. As a result, the total electric charge of a mediumgroup having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn-based oxide, alarge group can be formed using not only the medium group illustrated inFIG. 16A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 16A.

Specifically, by repeating the large group illustrated in FIG. 16B, an

In—Ga—Zn-based oxide can be obtained. Note that a layered structure ofthe obtained In—Ga—Zn-based oxide can be expressed as a compositionformula, InGaO₃(ZnO)_(n) (n is a natural number).

This embodiment can be implemented in combination with any otherembodiment.

Embodiment 3

In the light-emitting device according to one embodiment of the presentinvention, a color filter method can be employed, in which full-colorimages are displayed by using a combination of a color filter and alight-emitting element that emits light of a single color such as white.Alternatively, it is possible to employ a method in which full-colorimages are displayed by using a plurality of light-emitting elementsthat emit light of different hues. This method is referred to as aseparate coloring method because EL layers each provided between a pairof electrodes in a light-emitting element are separately colored withcorresponding colors.

In the separate coloring method, in general, EL layers are separatelyapplied by evaporation with the use of a mask such as a metal mask;therefore, the size of pixels depends on the accuracy of separatecoloring of the EL layers by evaporation. On the other hand, unlike theseparate coloring method, EL layers do not need to be separately appliedin the color filter method. Accordingly, pixels can be downsized moreeasily than in the separate coloring method; thus, a high-definitionpixel portion can be provided.

A light-emitting device includes, in its category, a bottom-emissionlight-emitting device in which light emitted from a light-emittingelement is extracted from an element substrate, over which a transistoris formed; and a top-emission light-emitting device in which lightemitted from a light-emitting element is extracted from a side oppositeto an element substrate. In the top-emission structure, light emittedfrom a light-emitting element is not blocked by an element such as awiring, a transistor, or a storage capacitor, so that the efficiency oflight extraction from a pixel can be made higher than in thebottom-emission structure. Therefore, the top-emission structure canachieve high luminance even when the amount of current supplied to alight-emitting element is lowered, and thus is advantageous in improvingthe lifetime of the light-emitting element.

The light-emitting device according to one embodiment of the presentinvention may have a microcavity (micro optical resonator) structure inwhich light emitted from an EL layer resonates within a light-emittingelement. With the microcavity structure, light having a specificwavelength can be extracted from the light-emitting element with highefficiency, so that the luminance and the color purity of the pixelportion can be increased.

FIG. 10 is an example of a cross-sectional view of pixels. FIG. 10illustrates part of a cross section of a pixel corresponding to red,part of a cross section of a pixel corresponding to green, and part of across section of a pixel corresponding to blue.

Specifically, FIG. 10 illustrates a pixel 140 r corresponding to red, apixel 140 g corresponding to green, and a pixel 140 b corresponding toblue. The pixel 140 r, the pixel 140 g, and the pixel 140 b include ananode 715 r, an anode 715 g, and an anode 715 b, respectively. Theanodes 715 r, 715 g, and 715 b included in the pixels 140 r, 140 g, and140 b are provided over an insulating film 750 formed over a substrate740.

A bank 730 formed using an insulating film is provided over the anodes715 r, 715 g, and 715 b. The bank 730 has openings, where parts of theanodes 715 r, 715 g, and 715 b are exposed. An EL layer 731 and acathode 732 that transmits visible light are stacked in this order overthe bank 730 so as to cover the above exposed parts.

A portion where the anode 715 r, the EL layer 731, and the cathode 732overlap with one another corresponds to a light-emitting element 741 rcorresponding to red. A portion where the anode 715 g, the EL layer 731,and the cathode 732 overlap with one another corresponds to alight-emitting element 741 g corresponding to green. A portion where theanode 715 b, the EL layer 731, and the cathode 732 overlap with oneanother corresponds to a light-emitting element 741 b corresponding toblue.

A substrate 742 is provided to face the substrate 740 with thelight-emitting elements 741 r, 741 g, and 741 b placed therebetween. Acoloring layer 743 r corresponding to the pixel 140 r, a coloring layer743 g corresponding to the pixel 140 g, and a coloring layer 743 bcorresponding to the pixel 140 b are provided on the substrate 742. Thecoloring layer 743 r is a layer whose transmittance of light in awavelength range corresponding to red is higher than that of light inother wavelength ranges. The coloring layer 743 g is a layer whosetransmittance of light in a wavelength range corresponding to green ishigher than that of light in other wavelength ranges. The coloring layer743 b is a layer whose transmittance of light in a wavelength rangecorresponding to blue is higher than that of light in other wavelengthranges.

An overcoat 744 is provided on the substrate 742 so as to cover thecoloring layers 743 r, 743 g, and 743 b. The overcoat 744 transmitsvisible light, is provided for protecting the coloring layers 743 r, 743g, and 743 b, and is preferably formed using a resin material with whichplanarity can be improved. The coloring layers 743 r, 743 g, and 743 band the overcoat 744 may be collectively regarded as a color filter, oreach of the coloring layers 743 r, 743 g, and 743 b may be regarded as acolor filter.

In FIG. 10, a conductive film 745 r with high visible-light reflectanceand a conductive film 746 r with higher visible-light transmittance thanthe conductive film 745 r are stacked in this order to be used as theanode 715 r. A conductive film 745 g with high visible-light reflectanceand a conductive film 746 g with higher visible-light transmittance thanthe conductive film 745 g are stacked in this order to be used as theanode 715 g. The conductive film 746 g has a smaller thickness than theconductive film 746 r. A conductive film 745 b with high visible-lightreflectance is used as the anode 715 b.

Thus, in the light-emitting device in FIG. 10, the optical path lengthof light emitted from the EL layer 731 in the light-emitting element 741r can be adjusted by the distance between the conductive film 745 r andthe cathode 732. The optical path length of light emitted from the ELlayer 731 in the light-emitting element 741 g can be adjusted by thedistance between the conductive film 745 g and the cathode 732. Theoptical path length of light emitted from the EL layer 731 in thelight-emitting element 741 b can be adjusted by the distance between theconductive film 745 b and the cathode 732.

In one embodiment of the present invention, a microcavity structure maybe employed, in which the above optical path lengths are adjusted inaccordance with the wavelengths of light emitted from the light-emittingelements 741 r, 741 g, and 741 b so that light emitted from the EL layer731 resonates within each light-emitting element.

When the microcavity structure is applied to the light-emitting deviceaccording to one embodiment of the present invention, light with awavelength corresponding to red among the light emitted from thelight-emitting element 741 r resonates in the microcavity structure toenhance its intensity. Consequently, the color purity and luminance ofred light obtained through the coloring layer 743 r are increased. Lightwith a wavelength corresponding to green among the light emitted fromthe light-emitting element 741 g resonates in the microcavity structureto enhance its intensity, and the color purity and luminance of greenlight obtained through the coloring layer 743 g are increased as aresult. Light with a wavelength corresponding to blue among the lightemitted from the light-emitting element 741 b resonates in themicrocavity structure to enhance its intensity; consequently, the colorpurity and luminance of blue light obtained through the coloring layer743 b are increased.

Note that although pixels corresponding to three colors of red, green,and blue are shown in FIG. 10, one embodiment of the present inventionis not limited to this structure. In one embodiment of the presentinvention, a combination of four colors of red, green, blue, and yellowor a combination of three colors of cyan, magenta, and yellow may beused. Alternatively, it is possible to use a combination of six colorsof pale red, pale green, pale blue, deep red, deep green, and deep blue,or a combination of six colors of red, green, blue, cyan, magenta, andyellow.

Note that colors that can be expressed using pixels of red, green, andblue, for example, are limited to colors existing in the triangle madeby the three points on the chromaticity diagram which correspond to theemission colors of the respective pixels. Therefore, by additionallyproviding a light-emitting element of a color existing outside thetriangle on the chromaticity diagram as in the case where pixels of red,green, blue, and yellow are used, the range of the colors that can beexpressed in the light-emitting device can be expanded and the colorreproducibility can be enhanced as a result.

In FIG. 10, the conductive film 745 b with high visible-lightreflectance is used as the anode in the light-emitting element 741 bwhich emits light with the shortest wavelength X among thelight-emitting elements 741 r, 741 g, and 741 b, and the conductivefilms 746 r and 746 g having different thicknesses are used in the otherlight-emitting elements 741 r and 741 g; thus, the optical path lengthsare adjusted. In one embodiment of the present invention, a conductivefilm with high visible-light transmittance, like the conductive films746 r and 746 g, may be provided over the conductive film 745 b withhigh visible-light reflectance also in the light-emitting element 741 bwhich emits light with the shortest wavelength X. However, it ispreferable to use the conductive film 745 b with high visible-lightreflectance as the anode of the light-emitting element 741 b which emitslight with the shortest wavelength X as shown in FIG. 10, because thefabrication process of the anode can be simplified as compared to thecase of using a conductive film with high visible-light transmittancefor the anodes of all the light-emitting elements 741 r, 741 g, and 741b.

Note that the work function of the conductive film 745 b with highvisible-light reflectance is often smaller than those of the conductivefilms 746 r and 746 g with high visible-light transmittance.Accordingly, in the light-emitting element 741 b which emits light withthe shortest wavelength X, holes are less likely to be injected from theanode 715 b into the EL layer 731 than in the light-emitting elements741 r and 741 g, resulting in low emission efficiency. In view of this,in one embodiment of the present invention, a composite material thatcontains a substance having a high hole-transport property and asubstance having an acceptor property (electron-accepting property) withrespect to the substance having a high hole-transport property ispreferably used for part of the EL layer 731 that is in contact with theconductive film 745 b with high visible-light reflectance in thelight-emitting element 741 b which emits light with the shortestwavelength X. When the above composite material is formed to be incontact with the anode 715 b, holes can be easily injected from theanode 715 b into the EL layer 731, so that the emission efficiency ofthe light-emitting element 741b can be increased.

Examples of the substance having an acceptor property are7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F₄-TCNQ),chloranil, a transition metal oxide, and oxides of metals belonging toGroups 4 to 8 in the periodic table. Specifically, vanadium oxide,niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferablebecause of their high acceptor properties. Among these, molybdenum oxideis particularly preferable since it is stable in the air, has a lowhygroscopic property, and is easily treated.

As the substance having a high hole-transport property used for thecomposite material, any of a variety of compounds such as an aromaticamine compound, a carbazole derivative, aromatic hydrocarbon, or a highmolecular weight compound (e.g., an oligomer, a dendrimer, or a polymer)can be used. The organic compound used for the composite material ispreferably an organic compound having a high hole-transport property.Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs orhigher is preferably used. Note that any other substance may also beused as long as its hole-transport property is higher than itselectron-transport property.

The conductive films 745 r, 745 g, and 745 b having high visible-lightreflectance can be formed with a single layer or a stack using aluminum,silver, or an alloy containing such a metal material, for example.Alternatively, the conductive films 745 r, 745 g, and 745 b may beformed by stacking a conductive film with high visible-light reflectanceand a thin conductive film (preferably with a thickness of 20 nm orless, further preferably 10 nm or less). For example, a thin titaniumfilm or a thin molybdenum film may be stacked over a conductive filmwith high visible-light reflectance to form the conductive film 745 b,in which case an oxide film can be prevented from being formed on asurface of the conductive film with high visible-light reflectance(e.g., aluminum, an alloy containing aluminum, or silver).

The conductive films 746 r and 746 g with high visible-lighttransmittance can be formed using, for example, indium oxide, tin oxide,zinc oxide, indium tin oxide, or indium zinc oxide.

The cathode 732 can be formed, for example, by stacking a conductivefilm thin enough to transmit light (preferably with a thickness of 20 nmor less, further preferably 10 nm or less) and a conductive filmincluding a conductive metal oxide. The conductive film thin enough totransmit light can be formed with a single layer or a stack usingsilver, magnesium, an alloy containing such a metal material, or thelike. Examples of the conductive metal oxide are indium oxide, tinoxide, zinc oxide, indium tin oxide, indium zinc oxide, and any of thesemetal oxide materials containing silicon oxide.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 4

In this embodiment, a bottom-emission structure, a top-emissionstructure, and a dual-emission structure will be described. In thedual-emission structure, light from a light-emitting element isextracted from the element substrate side and the side opposite to theelement substrate.

FIG. 11A is a cross-sectional view of a pixel in which light emittedfrom a light-emitting element 6033 is extracted from an anode 6034 side.A transistor 6031 is covered with an insulating film 6037, and a bank6038 having an opening is formed over the insulating film 6037. In theopening of the bank 6038, the anode 6034 is partially exposed, and theanode 6034, an EL layer 6035, and a cathode 6036 are stacked in thisorder in the opening.

The anode 6034 is formed using a material through which light passeseasily or formed to a thickness such that light passes through the anode6034 easily. The cathode 6036 is formed using a material through whichlight is difficult to pass or formed to a thickness such that light isdifficult to pass through the cathode 6036. Accordingly, it is possibleto obtain a bottom-emission structure in which light is extracted fromthe anode 6034 side as indicated by an outline arrow.

FIG. 11B is a cross-sectional view of a pixel in which light emittedfrom a light-emitting element 6043 is extracted from a cathode 6046side. A transistor 6041 is covered with an insulating film 6047, and abank 6048 having an opening is formed over the insulating film 6047. Inthe opening of the bank 6048, an anode 6044 is partly exposed, and theanode 6044, an EL layer 6045, and the cathode 6046 are stacked in thisorder in the opening.

The anode 6044 is formed using a material through which light isdifficult to pass or formed to a thickness such that light is difficultto pass through the anode 6044. The cathode 6046 is formed using amaterial through which light passes easily or formed to a thickness suchthat light passes through the cathode 6046 easily.

Accordingly, it is possible to obtain a top-emission structure in whichlight is extracted from the cathode 6046 side as indicated by an outlinearrow.

FIG. 11C is a cross-sectional view of a pixel in which light emittedfrom a light-emitting element 6053 is extracted from an anode 6054 sideand a cathode 6056 side. A transistor 6051 is covered with an insulatingfilm 6057, and a bank 6058 having an opening is formed over theinsulating film 6057. In the opening of the bank 6058, the anode 6054 ispartially exposed, and the anode 6054, an EL layer 6055, and the cathode6056 are stacked in this order in the opening.

The anode 6054 and the cathode 6056 are formed using a material throughwhich light passes easily or formed to a thickness such that lightpasses through the anode 6054 and the cathode 6056 easily. Accordingly,it is possible to obtain a dual-emission structure in which light isextracted from the anode 6054 side and the cathode 6056 side asindicated by outline arrows.

For the electrodes serving as the anode and the cathode, any of metals,alloys, electrically conductive compounds, and mixtures thereof can beused, for example. Specific examples are indium oxide-tin oxide (ITO:indium tin oxide), indium oxide-tin oxide containing silicon or siliconoxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti). Otherexamples are elements belonging to Group 1 or Group 2 of the periodictable, for instance, an alkali metal such as lithium (Li) and cesium(Cs), an alkaline earth metal such as calcium (Ca) and strontium (Sr),magnesium (Mg), an alloy containing such an element (e.g., MgAg andAlLi), a rare earth metal such as europium (Eu) and ytterbium (Yb), analloy containing such an element, and graphene. The electrodes areformed using materials selected from the above as appropriate and formedto have an optimal thickness, thereby achieving a bottom-emissionstructure, a top-emission structure, or a dual-emission structure.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 5

FIG. 12 is an example of a perspective view of the light-emitting deviceaccording to one embodiment of the present invention.

The light-emitting device illustrated in FIG. 12 includes a panel 1601,a circuit board 1602, and connection portions 1603. The panel 1601includes a pixel portion 1604 including a plurality of pixels, a scanline driver circuit 1605 that selects pixels per row, and a signal linedriver circuit 1606 that controls input of an image signal to the pixelsin a selected row. Specifically, signals input to the wirings G1 to G3are generated in the scan line driver circuit 1605.

Various signals and power supply potentials are input to the panel 1601through the connection portions 1603 from the circuit board 1602. Forthe connection portion 1603, a flexible printed circuit (FPC) can beused, for example. In the case where a COF tape is used as theconnection portion 1603, part of the circuit in the circuit board 1602or part of the scan line driver circuit 1605 or the signal line drivercircuit 1606 included in the panel 1601 may be formed on a chipseparately prepared, and the chip may be connected to a COF tape by aCOF (chip on film) method.

This embodiment can be implemented in combination with any otherembodiment.

Embodiment 6

The light-emitting device according to one embodiment of the presentinvention can be used for display devices, personal computers, and imagereproducing devices provided with recording media (typically, devicesthat reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Other examples of electronic devices that can include the light-emittingdevice according to one embodiment of the present invention are mobilephones, game machines including portable game machines, personal digitalassistants, e-book readers, cameras such as video cameras and digitalstill cameras, goggle-type displays (head mounted displays), navigationsystems, audio reproducing devices (e.g., car audio systems and digitalaudio players), copiers, facsimiles, printers, multifunction printers,automated teller machines (ATM), and vending machines. FIGS. 13A to 13Eillustrate specific examples of these electronic devices.

FIG. 13A illustrates a portable game machine including a housing 5001, ahousing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, a speaker 5006, an operation key 5007, a stylus 5008,and the like. The light-emitting device according to one embodiment ofthe present invention can be used as the display portion 5003 or thedisplay portion 5004. By using the light-emitting device according toone embodiment of the present invention as the display portion 5003 orthe display portion 5004, a portable game machine with high imagequality can be provided. Note that although the portable game machine inFIG. 13A includes the two display portions 5003 and 5004, the number ofdisplay portions included in the portable game machine is not limited totwo.

FIG. 13B illustrates a display device including a housing 5201, adisplay portion 5202, a support base 5203, and the like. Thelight-emitting device according to one embodiment of the presentinvention can be used as the display portion 5202. By using thelight-emitting device according to one embodiment of the presentinvention as the display portion 5202, a display device with high imagequality can be provided. Note that a display device includes, in itscategory, any display device for displaying information, such as displaydevices for personal computers, TV broadcast reception, andadvertisement.

FIG. 13C illustrates a laptop personal computer including a housing5401, a display portion 5402, a keyboard 5403, a pointing device 5404,and the like. The light-emitting device according to one embodiment ofthe present invention can be used as the display portion 5402. By usingthe light-emitting device according to one embodiment of the presentinvention as the display portion 5402, a laptop personal computer withhigh image quality can be provided.

FIG. 13D illustrates a personal digital assistant including a housing5601, a display portion 5602, operation keys 5603, and the like. In thepersonal digital assistant in FIG. 13D, a modem may be incorporated inthe housing 5601. The light-emitting device according to one embodimentof the present invention can be used as the display portion 5602. Byusing the light-emitting device according to one embodiment of thepresent invention as the display portion 5602, a personal digitalassistant with high image quality can be provided.

FIG. 13E illustrates a mobile phone including a housing 5801, a displayportion 5802, an audio input portion 5803, an audio output portion 5804,operation keys 5805, a light-receiving portion 5806, and the like. Lightreceived in the light-receiving portion 5806 is converted intoelectrical signals, whereby external images can be loaded. Thelight-emitting device according to one embodiment of the presentinvention can be used as the display portion 5802. By using thelight-emitting device according to one embodiment of the presentinvention as the display portion 5802, a mobile phone with high imagequality can be provided.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 7

In this embodiment, a gate voltage Vgs of the transistor 11 in the thirdperiod of the operation of the pixel 100 in FIG. 1A, which is describedin Embodiment 1, was obtained by calculation.

The calculation was performed under Condition A and Condition B withdifferent values of the potential V0 of the wiring IL. Table 1 showsspecific potentials of the wirings under Condition A and Condition B. Apotential GVDD corresponds to a high-level potential applied to thewirings G1, G2, and G3. A potential GVSS corresponds to a low-levelpotential applied to the wirings G1, G2, and G3. Note that in Table 1,the potential Vcat is 0 V, and values of the potential Vdata, thepotential Vano, the potential V0, the potential GVDD, and the potentialGVSS are represented by a potential difference with respect to thepotential Vcat.

TABLE 1 Condition A Condition B Vth −3 V to 3 V −3 V to 3 V Vdata 10 Vto 15 V 14 V to 19 V V0 10 V 14 V Vano 14 V 14 V Vcat  0 V  0 V GVDD/ 20V/0 V 25 V/0 V GVSS

As for the channel width W to channel length L ratio of the transistorsin the calculation, W/L of the transistor 11 was 3 μm/9 μm and W/L ofthe transistors 12 to 15 was 3 μm/3 μm. Assuming that a region A is aregion where the conductive film functioning as the source or the drainis in contact with the semiconductor film in all the transistorsincluded in the pixel 100 in FIG. 1A, the length (Lov) in the channellength direction of a region in which the region A overlaps with aregion where the gate electrode is formed was 1.5 μm.

In the third period, the gate voltage Vgs of the transistor 11 wasVdata−V0+Vth as illustrated in FIG. 3C. Thus, the equationVgs−Vth=Vdata−V0 holds in the pixel 100 in FIG. 1A, so that Vgs−Vth isideally constant regardless of the value of the threshold voltage Vth.

FIG. 17 shows Vgs−Vth obtained by the calculation under Condition A. InFIG. 17, the horizontal axis represents the threshold voltage Vth (V)and the vertical axis represents Vgs−Vth (V). It is found from FIG. 17that the values of Vgs−Vth are almost constant even when the thresholdvoltage Vth is changed, and the variation in Vgs−Vth is limited to lessthan about 25% to 30%.

FIG. 18 shows Vgs−Vth obtained by the calculation under Condition B. InFIG. 18, the horizontal axis represents the threshold voltage Vth (V)and the vertical axis represents Vgs−Vth (V). In FIG. 18, the values ofVgs−Vth are almost constant when the threshold voltage Vth is positive.In contrast, when the threshold voltage Vth is negative, Vgs−Vth islarger as the threshold voltage Vth of negative polarity is higher,which means Vgs−Vth depends on the threshold voltage Vth.

The results of the calculation prove that in the light-emitting deviceaccording to one embodiment of the present invention, the gate voltageVgs of the transistor 11 can be set at a value to which the thresholdvoltage Vth of the transistor 11 is added, even when the transistor 11is a normally-on transistor, that is, when the threshold voltage Vth isnegative.

This embodiment can be implemented in combination with any otherembodiment.

EXPLANATION OF REFERENCE

-   11: transistor, 12: transistor, 13: transistor, 14: transistor, 15:    transistor, 16: capacitor, 17: light-emitting element, 100: pixel,    140 b: pixel, 140 g: pixel, 140 r: pixel, 715 b: anode, 715 g:    anode, 715 r: anode, 730: bank, 731: EL layer, 732: cathode, 740:    substrate, 741 b: light-emitting element, 741 g: light-emitting    element, 741 r: light-emitting element, 742: substrate, 743 b:    coloring layer, 743 g: coloring layer, 743 r: coloring layer, 744:    overcoat, 745 b: conductive film, 745 g: conductive film, 745 r:    conductive film, 746 g: conductive film, 746 r: conductive film,    750: insulating film, 800: substrate, 801: conductive film, 802:    gate insulating film, 803: semiconductor film, 804: conductive film,    805: conductive film, 806: semiconductor film, 807: conductive film,    808: conductive film, 809: conductive film, 810: conductive film,    811: semiconductor film, 812: conductive film, 813: semiconductor    film, 814: conductive film, 815: conductive film, 816: conductive    film, 817: semiconductor film, 818: conductive film, 819: conductive    film, 820: insulating film, 821: insulating film, 822: conductive    film, 823: contact hole, 824: insulating film, 825: EL layer, 826:    conductive film, 900: substrate, 901: semiconductor film, 902: gate    insulating film, 903: conductive film, 904: conductive film, 905:    conductive film, 906: semiconductor film, 907: conductive film, 908:    conductive film, 909: conductive film, 911: conductive film, 912:    semiconductor film, 913: conductive film, 914: conductive film, 915:    conductive film, 916: conductive film, 917: conductive film, 920:    insulating film, 921: conductive film, 922: contact hole, 923:    insulating film, 924: EL layer, 925: conductive film, 1601: panel,    1602: circuit board, 1603: connection portion, 1604: pixel portion,    1605: scan line driver circuit, 1606: signal line driver circuit,    5001: housing, 5002: housing, 5003: display portion, 5004: display    portion, 5005: microphone, 5006: speaker, 5007: operation key, 5008:    stylus, 5201: housing, 5202: display portion, 5203: support base,    5401: housing, 5402: display portion, 5403: keyboard, 5404: pointing    device, 5601: housing, 5602: display portion, 5603: operation key,    5801: housing, 5802: display portion, 5803: audio input portion,    5804: audio output portion, 5805: operation key, 5806:    light-receiving portion, 6031: transistor, 6033: light-emitting    element, 6034: anode, 6035: EL layer, 6036: cathode, 6037:    insulating film, 6038: bank, 6041: transistor, 6043: light-emitting    element, 6044: anode, 6045: EL layer, 6046: cathode, 6047:    insulating film, 6048: bank, 6051: transistor, 6053: light-emitting    element, 6054: anode, 6055: EL layer, 6056: cathode, 6057:    insulating film, 6058: bank

This application is based on Japanese Patent Applications serial No.2011-161103 and No. 2011-259828 filed with Japan Patent Office on Jul.22, 2011 and Nov. 29, 2011, respectively, the entire contents of whichare hereby incorporated by reference.

1. (canceled)
 2. A method for driving a light-emitting devicecomprising: a first wiring; a second wiring; a third wiring; and a pixelcomprising: a transistor; a first switch; a second switch; a thirdswitch; a fourth switch; a capacitor; and a light-emitting element,wherein the first wiring and a first electrode of the capacitor areelectrically connected to each other through the first switch, wherein asecond electrode of the capacitor is electrically connected to a firstterminal of the transistor and an anode of the light-emitting element,wherein the second wiring and a gate of the transistor are electricallyconnected to each other through the second switch, wherein the firstelectrode of the capacitor and the gate of the transistor areelectrically connected to each other through the third switch, andwherein the first terminal of the transistor and the third wiring areelectrically connected to each other through the fourth switch, themethod comprising the steps of: supplying an anode potential to a secondterminal of the transistor; supplying a cathode potential to a cathodeof the light-emitting element; supplying a third potential to the thirdwiring; turning off the first switch, the second switch, and the thirdswitch in a first period; and turning on the fourth switch in the firstperiod, wherein the anode potential is higher than the sum of athreshold voltage of the light-emitting element and the cathodepotential, and wherein the third potential is lower than the sum of thecathode potential and the threshold voltage of the light-emittingelement.
 3. The method for driving the light-emitting device accordingto claim 2, further comprising the steps of: supplying a first potentialcorresponding to an image signal to the first wiring; supplying a secondpotential to the second wiring; turning on the first switch and thesecond switch in a second period; and turning off the third switch andthe fourth switch in the second period, wherein the second potential ishigher than the sum of the cathode potential, a threshold voltage of thetransistor, and the threshold voltage of the light-emitting element, andwherein the second potential is lower than the sum of the anodepotential and the threshold voltage of the transistor.
 4. The method fordriving the light-emitting device according to claim 3, furthercomprising the steps of: turning off the first switch, the secondswitch, and the fourth switch in a third period; and turning on thethird switch in the third period.
 5. The method for driving thelight-emitting device according to claim 3, wherein the fourth switch isturned off after the first switch and the second switch are turned on inthe second period.
 6. The method for driving the light-emitting deviceaccording to claim 5, wherein the light-emitting element comprises theanode over the transistor, an EL layer over the anode, and the cathodeover the EL layer.
 7. A light-emitting device comprising: a firstwiring; a second wiring; a third wiring; and a pixel comprising: atransistor; a first switch; a second switch; a third switch; a fourthswitch; a capacitor; and a light-emitting element, wherein the firstwiring and a first electrode of the capacitor are electrically connectedto each other through the first switch, wherein a second electrode ofthe capacitor is electrically connected to a first terminal of thetransistor and an anode of the light-emitting element, wherein thesecond wiring and a gate of the transistor are electrically connected toeach other through the second switch, wherein the first electrode of thecapacitor and the gate of the transistor are electrically connected toeach other through the third switch, and wherein the first terminal ofthe transistor and the third wiring are electrically connected to eachother through the fourth switch.
 8. The light-emitting device accordingto claim 7, wherein the transistor comprises an oxide semiconductor filmcomprising a channel formation region.
 9. The light-emitting deviceaccording to claim 7, wherein the transistor is an n-channel transistor.10. The light-emitting device according to claim 9, wherein the first tofourth switches are n-channel transistors.
 11. The light-emitting deviceaccording to claim 9, wherein the light-emitting element comprises theanode over the transistor, an EL layer over the anode, and a cathodeover the EL layer.